I've had some problems with textures like this before I went to a 3 computer setup.
and it occasionally happens on my "worst" computer after I zone a few times in and out of heavy traffic zones.

Somehow I've never seen other textures filling the gaps up tho, usually everything turned black.
(which improved performance amazingly at that time,
I wish there was some option to turn off all environmental textures, so the performance was there to stay